This invention relates to a waste tank for a vacuum sewage system used in conjunction with aircraft vacuum toilet systems.
As disclosed in U.S. Pat. No. 5,026,407, anyone who has ever made a relatively long flight aboard a commercial passenger jet is probably familiar with their toilets. Flushing these devices results in toilet fluids and solid wastes being drawn from a toilet bowl down a waste line. Unlike conventional toilels, where waste exits the toilet bowl via a circular water flow that carries the waste through a bottom outlet, the toilets in the most recent passenger jet models are vacuum toilets that rely upon suction for removing waste. This creates the familiar sucking sound that accompanies flushing this particular toilet. A common attribute of such systems is that a flushing airflow is created by venting toilets externally of the aircraft. This is accomplished by opening a valve, which creates an airflow path from a given toilet bowl to the outside or ambient atmosphere via a waste line and tank system. The pressure differential between the toilet cabin and the outside is what actually generates the airflow. At certain low elevations, where the pressure differential is not great, a vacuum blower is employed to assist or augment the natural pressure differential between inside and outside the aircraft.
Of course, the solid and liquid waste in the toilet is not simply dumped outside the aircraft. Instead, it is separated from the airflow, and deposited in a waste tank prior to venting the air overboard.
Typically, the airflow and entrained waste travel from the toilet to the waste tank via conventional pipes or lines. The conventional waste tank has one or more waste inlets configured to direct the flow circumferentially in a clockwise direction around the tank's interior, but at a level that is above and parallel to the level of waste already in the tank. As a result, a combination of centrifugal forces and gravity cause separation of much of the entrained matter from the airflow, and it simply drops downwardly into the tank. Some entrained matter remains with the airflow and is removed via a separator as it exits the tank. This device is normally positioned inside the top portion of the tank.
However, when the tank reaches a level approaching the full liquid level of the tank, the kinetic energy of the sewage entering the tank causes splashing and creates waves which interact with the airflow causing more liquid to be entrained in the airflow than can be handled efficiently by the separator. In this case liquid is exhausted to the atmosphere and forms ice on the aircraft exterior which then could break off and cause serious problems when it strikes the ground.
The waste tank system described above has sensors for detecting the level of waste inside the tank. These sensors have faces that are positioned at a certain vertical height along the tank's inner wall, and provide an electrical signal indicating a full tank in response to contact with the waste as its level rises. In the full tank condition, the level sensors remove power from all toilets connected to the tank.
The above-described tank inlet arrangement, which a circular flow motion inside the tank, also creates a problem in that it tends to coat the waste level sensor faces with solid and liquid waste. This has been known to cause the sensors to emit signals falsely indicating a full tank, resulting in unnecessary shutdown of the toilet. This naturally results in a serious inconvenience for the passengers.
The typical waste tank system also has one or more rinse nozzles that protrude into the tank. These are connectable to an external source of clean water for periodically rinsing and/or cleaning the tank during aircraft maintenance intervals. They also tend to be coated by incoming waste from tank inlets which can clog them.